This invention relates to a process that utilizes a laser pulse to etch selected sections of a metal layer on a non-metallic substrate to form a patterned metal feature, such as a circuit trace or the like. More particularly, this invention relates to a laser etch-back process that forms a metal layer having a thick section and a thin section, and thereafter uniformly irradiates the layer with a laser pulse to remove the thin section without disturbing the thick section.
In the manufacture of a printed circuit board or the like, a metal circuit trace is formed on a non-metallic substrate to provide electrical connection between attached components. The trace is typically formed of copper having high electrical conductivity, whereas the substrate is formed of a dielectric material such as polymer resin. The trace extends in a predetermined pattern that is surrounded by bare substrate that electrically insulates segments of the trace. Thus, the substrate includes a first region to which the trace is applied and a second, adjacent region at which the substrate is exposed.
One method for forming a trace comprises selectively electroplating metal onto the desired region. Because the substrate is non-conductive, a thin metal film, preferably formed also of copper, is first applied to the substrate to conduct electrical current that is essential for plating. This initial metal film is deposited onto the entire area, including both the first region and the second region, to improve distribution of the current and obtain more uniform plating. A photolithographic mask is applied to the metal film and has openings that expose the metal film at the first region to limit plating to the desired pattern of the trace. Following plating, the mask is removed, thereby exposing the thin film in the second region for removal by a metal etching process. This etch-back is typically carried out using an aqueous etching solution. Because the etching solution also attacks the plated metal, the plating operation is extended to deposit surplus metal onto the first region that is sacrificed during etching. This adds to the processing time and material costs of the product. In addition, the solution etches exposed surfaces at a uniform rate, including side surfaces of the trace, which tends to produce undercutting.
It is known to remove unwanted metal using a laser beam to heat and vaporize the metal. Such laser ablation commonly involves focusing the beam on a very small area so that the pulse only irradiates the unwanted metal, and then scanning the substrate to progressively clear the region. This scanning process is time-consuming and requires care to aim the pulse to avoid the desired metal.
Therefore, there remains a need for an improved etch-back process that uses a laser beam for removing unwanted sections of a metal layer to define a feature, such as a circuit trace or the like, and that permits a relatively large area of a substrate to be concurrently treated with the laser beam to remove metal from one region without disturbing metal on an adjacent, co-irradiated region.